1. Field of the Invention
The present invention relates to an apparatus for controlling slip at drive wheels driven by an internal combustion engine.
2. Description of the Related Art
It is well known that slip is easily generated at the driving wheels of a vehicle provided with an high output power engine, when the vehicle is accelerated, because the engine output power is too high with respect to an adhesion force between the tires and the road surface whereby a grip of the tire is weakened. Therefore, a traction control system has been proposed, to prevent the generation of such a slip, wherein a sub-throttle valve is provided in an intake system of the internal combustion engine, which sub-throttle valve is operated independently of the depression of an accelerator pedal for controlling the amount of intake air, or wherein a throttle valve is constructed as a "link-less" type. In such a system, a slip is detected from a difference in a rotational speed of the driving wheels of the vehicle with respect to a rotational speed of the driven wheels, and the sub-throttle valve or link-less throttle valve is then closed to a degree of opening which is smaller than a degree of opening determined by the degree of depression of the accelerator pedal. As a result, the amount of intake air is reduced, and accordingly, the engine output torque is lowered, and thus the acceleration slip is controlled. See, for example, Japanese Unexamined Patent Publication No. 62-237047.
The sub-throttle valve is maintained at the fully open position during a normal operating period other than that wherein an acceleration slip is generated, so that a normal operation of the main throttle valve controlled by an accelerator pedal is not affected by the provision of the sub-throttle valve. This means that the sub-throttle valve is closed to a predetermined degree the instant the vehicle is accelerated, to prevent an acceleration slip. Nevertheless, since the sub-throttle valve is fully open just before the acceleration is commenced, a significant time is required before the sub-throttle valve can be closed to the predetermined degree. In particular, where an actuator for the sub-throttle valve is a stepper motor, to obtain a precise control of the opening of the degree of the sub-throttle valve, a quick movement of the sub-throttle valve to the desired position becomes much more difficult, due to a slow response speed characteristic inherent to the stepper motor.
Since the acceleration slip has a large value at the initial stage thereof, the traction control should be such that a large slip at the initial stage of the acceleration is effectively suppressed. In the prior art, as described above, the closing of the sub-throttle valve from a fully open condition to a desired degree of opening is commenced upon the detection of an acceleration slip, but a lengthy delay occurs in the closing of the sub-throttle valve, and therefore, the control of the engine output torque at the initial stage of the acceleration cannot be fully effected, and thus an ideal acceleration slip control cannot be obtained.
It would be easy to consider a control of the degree of the opening of the sub-throttle valve that will always conform to that of the main throttle valve, but this solution would result in a large torque at the step motor for operating the sub-throttle valve, which will damage the step motor after little use. Furthermore, there is some difficulty in controlling the sub-throttle valve to follow the main throttle valve while obtaining a traction control, and therefore, an ideal operation will not be always obtained, and thus an ideal acceleration slip control for reducing an excessive torque at the initial stage of the acceleration by a depression of the accelerator pedal cannot be obtained.